"After 40 years, I just feel it’s somebody else’s turn," Harkin said in a lengthy statement. "I don’t by any means plan to retire completely from public life at the end of this Congress. But I am going to make way for someone new in this Senate seat. I think that is right not just for me, but for Iowa, as well."

Harkin, who is 73 years old, serves as chairman of the Senate Health, Education, Labor and Pensions Committee as well as the Appropriations subcommittee that handles the budgets for many of the same agencies. He passed up a chance to become chairman of the full Appropriations Committee after the death of Sen. Daniel K. Inouye, D-Hawaii, late last year.

Harkin's retirement sets off what is expected to be a highly competitive open-seat race in the swing state — one that is likely to feature two or more House members. Rep. Bruce Braley, who was exploring a run for governor, now tops the list of possible Democratic Senate candidates. Meanwhile, Republicans await word from two House members, Reps. Steve King and Tom Latham, on whether they will pursue the open seat.

Braley has made no secret of his aspirations for higher office and has regularly attended events outside his district — including in Des Moines, the state's largest media market. But when it appeared Harkin would seek re-election, local Democratic officials publicly encouraged Braley to challenge Gov. Terry Brandstad, a Republican, instead in 2014.

No doubt, running for an open Senate seat is Braley's easier path to statewide office than challenging a sitting governor.

An outspoken conservative, King has openly discussed his interest in the Senate. In December, King told CQ Roll Call that he was thinking about running for Senate but was not actively pursuing a statewide campaign. He represented a safe GOP seat for most of his career, but King won his first competitive House race in November by defeating the state's former first lady, Christie Vilsack.

However, there's some concern among Iowa Republicans that King is too conservative to run statewide. Branstad told reporters last year, "I think Steve King, I just don't think he can do it in Eastern Iowa." The governor said he thought Latham would be a better statewide candidate.

So far, Latham has not indicated interest in running statewide. He's one of House Speaker John A. Boehner's best friends, plus Latham just won a competitive race against a fellow member in 2012. Still, throughout Latham's 10-term career, he's represented most of the counties in the state at some point, giving him an incredible platform to run statewide.

Latham didn't give any clues on Saturday about his political future.

"Congressman Latham respects Senator Harkin's decision," spokesman James Carstensen said in a statement. "He looks forward to continuing to working with him and the rest of the Iowa delegation for the best interests of the people of Iowa over the next two years."

“I’m 73. I’ve been here a long time, and I’m at that point in my life where I don’t feel I have to do what others want me to do or expect me to do,” Harkin said. “I want to do what I love, and I love my committee. I love the issues we deal with, and I’m still on Approps.”

Harkin is the third Senator this cycle to announce his retirement, following Sens. Saxby Chambliss, R-Ga., and Jay Rockefeller, D-W.Va. To see which members are retiring or looking to move up in 2014 see Roll Call's casualty list.